Semiconductor device and method of producing a high contrast identification mark

ABSTRACT

A seconductor device ( 50 ) includes a semiconductor die ( 20 ) having a first surface ( 14 ) for forming electronic circuitry. A coating layer ( 16 ) formed on a second surface ( 15 ) of the semiconductor die has a color that contrasts with the color of the semiconductor die. The coating layer is patterned to expose a portion of the second surface to reveal information pertaining to the semiconductor device. The coating layer is patterned by directing a radiation beam ( 30 ) such as a laser to selectively remove material from the coating layer.

BACKGROUND OF THE INVENTION

The present invention relates in general to semiconductor devices and,more particularly, to semiconductor devices having a backside coatingthat is patterned to provide information pertaining to thesemiconductor.

Electronics system manufacturers continue to demand lower costintegrated circuits and other semiconductor devices in order to reducethe cost of electronics systems. In response, many semiconductormanufacturers are providing unpackaged semiconductor dice or chips whichcan be mounted in a “flip-chip” fashion with the active die surfaceattached directly to system circuit boards. This approach reduces thedirect cost of the semiconductor devices and also improves theirperformance by reducing lead inductance and other parasitic elements.However, the “flip-chip” technique often increases the indirect costsbecause such an unpackaged semiconductor chip reveals little or noinformation pertaining to the type of die such as its part number ormanufacturer. In case of a system malfunction, it is difficult to tracea defect to a particular manufacturer or fabrication process.

To avoid this problem, some chips are fabricated with a thick backsidesurface coating whose surface is marked with information about thesemiconductor device to facilitate tracing the device to itsmanufacturer in the event of a defect. Other chips use a gold coatingwhich is etched to provide the desired pattern. However, theseapproaches result in the information having a low contrast and thereforelow visibility, in some cases even when viewed through a microscope orother visual tool. To compensate for the low contrast, the prior artcoatings are marked with large fonts, which reduces the amount ofinformation that can be provided, especially on a small die. As aresult, the ability to trace a defective semiconductor device to aspecific processing step is reduced, making it more difficult to preventfuture defects that could enhance the die yield and reliability toreduce the overall fabrication cost of the device and system.

Hence, there is a need for a semiconductor device and method ofproviding easily discernible information pertaining to the semiconductordevice in order to reduce the fabrication cost and increase thereliability of the semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a semiconductor wafer;

FIG. 2 is a cross-sectional view showing further detail of a portion ofthe wafer;

FIG. 3 is a cross-sectional view showing the portion of the wafer in afirst alternate embodiment;

FIG. 4 is a cross-sectional view showing the portion of the wafer in asecond alternate embodiment;

FIG. 5 is a cross-sectional view showing the portion of the wafer in athird alternate embodiment;

FIG. 6 is an exploded view of an electrical system including asemiconductor device formed with a packaged semiconductor die singulatedfrom the semiconductor wafer; and

FIG. 7 is a cross-sectional view of the semiconductor die housed in asemiconductor package.

DETAILED DESCRIPTION OF THE DRAWINGS

In the figures, elements having the same reference number have similarfunctionality.

FIG. 1 is an isometric view of a semiconductor wafer 10 fabricated tohave a plurality of semiconductor dice, one of which is designated assemiconductor substrate or die 20. An active or topside surface 14 isused to form electrical circuitry that may include transistors and/orother active devices. A wafer flat 12 provides information as to theconductivity type and crystallographic orientation of wafer 10. Acoating material 16 is applied to backside surface 15 after theelectrical circuitry has been formed.

FIG. 2 is a cross-sectional view showing a portion of wafer 10 includingsemiconductor die 20 in further detail. Note that semiconductor die 20is shown in a “flip-chip” orientation which is reversed from theorientation of FIG. 1.

A plurality of conductive bumps 18 are formed on topside surface 14 formaking electrical and mechanical connections between circuitry ontopside surface 14 and a system circuit board (not shown in FIG. 2).Conductive bumps 18 typically are formed with a low temperature solder,plated copper or another conductive material suitable for making thenecessary electrical and/or mechanical attachments without degrading theperformance of circuitry formed on semiconductor die 20.

After the electrical circuitry and conductive bumps 18 are formed,coating material 16 is applied to backside surface 15 to produce anopaque appearance whose color contrasts with that of semiconductor die20. Coating material 16 is selected to provide strong adhesion to theunderlying semiconductor material while meeting specified reliabilitystandards. Coating material 16 is formed with an organic materialbecause pigments can be added to produce a desired color and because theamount and type of filler material can be varied to provide a specifiedtemperature coefficient of expansion in order to minimize die or waferstress. Coating material 16 may have an elastic or compliantcharacteristic to farther reduce stress on semiconductor die 20 underspecified environmental and operating conditions. In one embodiment,coating material 16 comprises Hysol® to CNB876-36, a flexible epoxyresin with a silica filler material, available from Loctite Corporation,Industry, California. Coating material 16 may alternatively include apolymer.

Coating material 16 is configured, by adding pigments if necessary, tohave a color that contrasts with the color of the semiconductor materialvisible at surface 15. In an embodiment in which semiconductor die 20 isformed of silicon, which has a color that is described as off-white orbeige, coating material 16 is formed to have an opaque black color.Alternatively, coating material 16 may be formed to have virtually anyother color that provides a good visual contrast with the off-whitecolor of silicon.

Coating material 16 typically is applied in liquid form by screeningthrough a mesh whose height determines the film thickness. The screenfabric may be stainless steel or polyester with a mesh count betweenabout one hundred ten and about two hundred thirty. Coating material 16is applied to have a thickness in a range of about five to aboutthirty-seven micrometers. In one embodiment, coating material 16 isabout thirty micrometers thick, and is cured for thirty minutes at onehundred fifty degrees Celsius. Such a low curing temperature does notaffect the functionality of devices formed on semiconductor die 20, andis lower than the reflow or melting temperature of solder.

Information is marked on backside surface 15 by directing a ray 30 ofelectromagnetic radiation to selectively remove material from coatingmaterial 16 to form openings 21 that expose portions of backside surface15. In one embodiment, ray 30 comprises a laser beam which is programmedto produce openings 21 in the shape of alphanumeric characters or othersymbols that represent the desired information. Parameters such as thecurrent level of the laser device or beam intensity, pulse rate, beamdiameter and scan time of the laser beam can be adjusted if desired toensure that material from coating material 16 is completely removedwhile producing zero or only a minimal change to the structure ofsurface 15 of semiconductor die 20. Alternatively, openings 21 may beformed by applying a photoresist layer to coating material 16 andselectively etching to remove material. Alignment between topsidesurface 14 and backside surface 15, i.e., between openings 21 andsemiconductor die 20, is achieved with a standard alignment tool.

Because the film thickness of coating material 16 is less than aboutthirty-seven micrometers, highly readable alphanumeric characters can beproduced using character or font heights as small as about two-hundredfifty micrometers. Such small fonts allow more information to beprovided, and is of particular importance if semiconductor die 20 has asmall die area. The high contrast attributable to exposing portions ofbackside surface 15 makes the marked information discernible with littleor no magnification.

Semiconductor die 20 is bounded by a saw street 22 which is used tosingulate semiconductor die 20 from semiconductor wafer 10 after coatingmaterial 16 is cured and patterned.

FIG. 3 is a cross-sectional view showing the portion of wafer 10 in asecond embodiment. Elements of the second embodiment havecharacteristics similar to those described in FIG. 2, except that afteran opening 21A is formed in coating material 16, the intensity and/orpulse rate of ray 30 is increased to locally remove semiconductormaterial from backside surface 15 to form a recessed surface 17. Hence,when viewing opening 21A, both backside surface 15 and recessed surface17 are visible. The semiconductor material removed by ray 30 produces arougher texture for recessed surface 17 than the polished texture ofbackside surface 15. As a result, the perceived color of recessedsurface 17 is different from the perceived color of backside surface 15,and a marking scheme with three discernible contrasting colors isproduced, i.e., the colors of coating material 16, backside surface 15and recessed surface 17. The additional color provided by recessedsurface 17 provides an opportunity to convey additional informationwithout significantly increasing the fabrication cost.

FIG. 4 shows a cross-sectional view of the portion of wafer 10 in athird embodiment that provides a different contrast level or analternate color scheme. Coating material 16 has a multilayered orlaminated structure that includes a coating layer 25 of a first colorand a coating layer 26 of a second color that contrasts with the firstcolor. For example, the first and second colors may be white and black,respectively, green and white, or any other combination of contrastingcolors selected by adding pigments to coating layers 25–26 to conveyinformation such as a colored logo of a manufacturer.

Note that in this embodiment, openings 21 are formed to remove materialthrough coating layer 26 to expose a surface 27 of underlying coatinglayer 25 to achieve the desired contrast. Removal of material throughcoating layer 26 is achieved by adjusting the intensity of ray 30 and/orthe time during which it is applied. Coating layers 25–26 typically areapplied in successive laminations in a fashion similar to that describedin FIG. 2 and cured concurrently. Coating layers 25–26 each have athickness between about five micrometers and about eighteen micrometers.Alternatively, coating layers 25–26 are sequentially formed on a plasticsheet and subjected to a stage B cure so they can be handled withoutdamage. The sheet is brought into contact with backside surface 15,applied using a laminating tool, heated to promote adhesion and thencured prior to forming openings 21.

FIG. 5 shows a cross-sectional view of the portion of wafer 10 in afourth embodiment. In this embodiment, coating layer 16 comprises acoating layer 42 of a first color and a coating layer 40 of a secondcolor that contrasts both with the first color and the color ofsemiconductor die 20 shown by surface 15. Hence, after patterning, threecontrasting colors are visible, which can be used to provide a greateramount of desired information.

Coating layers 40 and 42 are applied in a fashion similar to thatdescribed in FIG. 4. Note that in this embodiment, openings 21 areformed by removing material through coating layer 40 to expose a surface28 of coating layer 42, while openings 23 are formed by removingmaterial from coating layer 40 to expose surface 15 of semiconductor die20. This multiple layer approach is readily extended to utilize three ormore coating layers of contrasting colors.

FIG. 6 shows an exploded view of an electrical system 70 including asystem circuit board 60 and a semiconductor device 50 formed withsemiconductor die 20 in accordance with the second alternate embodimentthat includes coating layers 40 and 42 as described in FIG. 4. Circuitboard 60 includes a mounting region 64 for mounting conductive bumps 18of semiconductor device 50 to a plurality of conductive bonding pads 62.

Information pertaining to semiconductor device 50 is provided asdescribed above by selectively removing material from coating layers 40and 42 to expose surfaces 28 and 15, thereby providing viewable symbolsand/or alphanumeric characters in multiple sharply contrasting colors.The alphanumeric characters shown in semiconductor device 50 are formedwith a height of about two hundred fifty micrometers.

Examples of this information are shown in FIG. 5, and can include theenduser's or system manufacturer's identity or custom part number toreduce the inventory or other cost of semiconductor device 50. Thesemiconductor manufacturer's logo or other identification can beprovided to facilitate communication in the event a defect is discoveredin semiconductor device 50. Wafer and die processing information such aslot, wafer and die identification as well as part numbers and/or serialnumbers allow a semiconductor manufacturer to trace semiconductor device50 to specific processing steps in order to localize the source of adefect. Many defects can be associated with a particular processing stepand often can be corrected by modifying the processing step, therebyimproving the reliability of similarly processed devices and reducingthe overall fabrication cost of the devices. In addition, informationuseful to an enduser or system manufacturer, such as the die orientationand/or location of a reference lead, i.e., pin “1”, can be provided aswell. In addition to alphanumeric characters, the enhanced contrastprovided by the above described structures is also suitable forproviding information in the form of machine readable symbols or barcodes.

FIG. 7 is a cross-sectional view of a packaged semiconductor device 80including semiconductor die 20 housed in a semiconductor package 82.Semiconductor die 20 is formed as a two-electrode device such as a diodewhose dimensions are less than about 1.5 millimeters on a side, and hasa vertical device structure with first and second electrodesrespectively formed on surfaces 14 and 15.

Semiconductor package 80 is a low profile, surface mount package havingfirst and second leads 83 and 84, respectively, arranged to form anearly chip scale package. Leads 83 and 84 are formed with a highlyconductive material such as copper or another metal. First lead 83 iselectrically coupled on surface 14 to the first electrode. Second lead84 includes an internal portion 85 electrically coupled on surface 15 tothe first electrode and an external portion 86 for making externalelectrical connections. Package 80 includes an organic encapsulant 82that protects semiconductor die 20 from environmental damage.

Encapsulant 82 is formed to a thickness less than about thirty-sevenmicrometers in a region overlying surface 87 of internal portion 85 tofunction as coating material 16. Coating material 16 is irradiated asdescribed above to remove material to form openings 21 that exposeportions of surface 87 to convey information pertaining to semiconductordevice 80. Encapsulant 82 is made with an organic material such as anepoxy into which are introduced pigments if desired to produce a colorthat contrasts sharply with the color of surface 87. Note that dependingon the application or type of package, openings 21 may be formed toexpose other package surfaces such as a surface of a lead frame, dieattach flag and the like.

In order to provide a significant amount of necessary information in thesmall amount of space available on package 80, openings 21 areconfigured with a small font size. Consequently, a sharp color contrastis particularly important in order to provide the high visibility neededto view the information.

In summary, a semiconductor device and method of providing informationabout the semiconductor device is described. A semiconductor die has afirst surface for forming electronic circuitry. A coating layer whosecolor contrasts with a color of the semiconductor die is disposed on asecond surface and patterned to expose the second surface to revealinformation in the form of symbols or alphanumeric characters pertainingto the semiconductor device. The coating layer is patterned by directingradiation such as a programmed laser beam to remove the material throughthe coating layer. Epoxy resins, polymers, or a variety of other organiccompositions can be used as coating materials to provide a desired levelof contrast. The coating layer may be applied by screening, laminating,spraying, pouring, grafting, or another method. The overall thickness ofthe coating layer is made less than about thirty-seven micrometers inorder to achieve a small font size, which increases the amount of highlydiscernible information that can be provided.

1. A semiconductor device, comprising: a semiconductor die having afirst surface for forming electronic circuitry; and a coating layerdisposed on a second surface of the semiconductor die to have a colorthat contrasts with a color of the semiconductor die, wherein thecoating layer is patterned to expose a portion of the second surface toreveal information pertaining to the semiconductor device, and whereinthe coating layer is patterned to form at least one opening that exposesthe second surface of the semiconductor die, and wherein thesemiconductor die has a recessed surface within the at least one openingthat has a color different from a color of the second surface.
 2. Thesemiconductor device of claim 1, wherein the coating layer is formed onthe second surface to a thickness less than about thirty-sevenmicrometers.
 3. The semiconductor device of claim 1, wherein the coatinglayer is formed with an organic material selected from the groupconsisting of an epoxy resin and a polymer.
 4. The semiconductor deviceof claim 1, wherein the coating layer is opaque and has a black color.5. The semiconductor device of claim 1, wherein the information includesan alphanumeric character having a height less than about three hundredmicrometers.
 6. The semiconductor device of claim 1, further comprisinga conductive bump formed on the first surface of the semiconductor diefor making an external electrical and mechanical connection.
 7. A methodof making a semiconductor device, comprising the steps of: disposing acoating layer over a surface of a semiconductor die; patterning thecoating layer to expose the surface to provide information about thesemiconductor device, and wherein the patterning step forms at least oneopening that exposes the surface of the semiconductor die; and forming arecessed portion in the semiconductor die through the at least oneopening so that the recessed portion has a color different from a colorof the surface.
 8. The method of claim 7, wherein the step of patterningincludes the step of selectively irradiating the coating layer.
 9. Themethod of claim 8, wherein the step of selectively irradiating includesthe step of directing a laser beam to remove material from the coatinglayer.
 10. The method of claim 9, wherein the step of directing includesthe step of forming an alphanumeric character with a font height lessthan about three hundred micrometers in the coating layer.
 11. Themethod of claim 7, wherein the step of disposing includes the step offorming the coating layer to a thickness of about thirty-sevenmicrometers.
 12. The method of claim 7, further comprising the step ofcuring the coating layer at a temperature less than about one hundredseventy-five degrees Celsius.
 13. The method of claim 7, wherein thestep of disposing includes the step of applying the coating layer to asemiconductor wafer, further comprising the step of singulating thesemiconductor wafer after the step of applying to form the semiconductordie.
 14. A semiconductor device, comprising: a semiconductor die formedwith an electrical component; and a semiconductor package having anorganic material for encapsulating the semiconductor die, whereinopenings are formed in the organic material to expose a conductivesurface of the semiconductor package with a contrasting color to providevisible information pertaining to the semiconductor device.
 15. Thesemiconductor device of claim 14, wherein the conductive surface isdisposed on a lead of the semiconductor package.